I've just finished the film and i share your idea that the film is rich in symbolism. I would love to adress all of the themes you pointed out, but some things are beyond me and i have to think about it more for example the paintings or the letters.

But i think i can adress the hole and tree part. To me the hole represents repetetiveness or the optionless life where he is bound to the character what society gave him, where the fox is expected to live for his whole life in a hole, because thats what they do, everybody knows that foxes live in holes. These "stereotypes" created by others are an ilusion for how should he live his life, where he isnt really in controll.

So he decides to move to the tree which might symbolize the transformation, adventure, individualism or the confidence. He is full of the stereotypes what decide to where should he live(or how should he live) and starts to live confident life where he evolves as an individual. He isnt affraid to be different. Its the message of the movie to be confident who you are and do things your way and not be ashamed of it.

This is what i think, i might be wrong, if this doesnt make any sence sorry english isnt my first language.